Multifunctional particles in the micrometer or sub-micrometer scale that exhibit two or more different properties are highly desirable for many important technological applications, ranging from catalysis to energy harvesting and transformation, multimodal imaging, detection, and simultaneous diagnosis and therapy. For example, microspheres embedded with magnetic iron oxide nanoparticles and fluorescent quantum dots have been widely studied as a multiple-mode imaging contrast agents combining magnetic resonance with optical detection and biological targeting. By carefully controlling the loading of quantum dots (QD), the obtained composite particles possess a dual function of optical encoding and magnetic separation. Replacing quantum dots with noble metal nanoparticles in such composites results in new types of multifunctional structures that are capable of magnetic resonance imaging and photothermal therapy. Magnetic materials have also been combined with nanocatalysts to form magnetically separable catalysts for the recovery and reuse of expensive catalysts after catalytic reactions, thus bridging the gap between heterogeneous and homogeneous catalysis.
Nanoparticle assembly represents a powerful approach that has been actively explored recently for producing bi-, tri-, and multifunctional materials in contrast to their limited single-component counterparts. By organizing different types of nanoparticles together, it not only allows the utilization of the size- and shape-dependent properties of individual nanoparticles, but also takes advantage of new properties resulting from the interactions between neighbors. Conventional assembly processes for multifunctional structures such as the well-known layer-by-layer (LbL) methods are mostly limited to hydrophilic nanoparticle systems because they heavily rely on electrostatic interactions. The direct linking nanoparticles through chemical bonds between surface ligands has also been attempted, but is limited to some special cases because most commonly used protecting ligands of nanoparticles do not contain additional active functional groups that allow further reactions.
Many technologically important high quality nanoparticles, especially semiconductors (such as CdSe, ZnSe, CdTe, and InP, InAs) and metal oxides (such as γ-Fe2O3, MnO, TiO2, ZrO2, CoFe2O4) are predominantly prepared through thermolytic routes by reacting inorganic precursors in organic solvents at high temperatures (preferably 150° C.-320° C.). The resulting nanostructures, however, retain the hydrophobic character of the organic ligands and, hence, are not soluble in water. As a result, it has been very difficult to assemble them directly into multifunctional nanostructures using means similar to those for water soluble particles. Typically, it is necessary to impose hydrophilic character on the nanoparticle surface and ensure water dispersibility (also bio-compatibility) by replacing the hydrophobic organic ligands with hydrophilic ones. However, ligand exchange processes usually involve several extra steps and in many cases are detrimental to the physical properties of the nanoparticles because the new hydrophilic ligands may not be able to effectively insulate the inorganic cores from the aqueous environment. For example, ligand exchange of hydrophobic trioctylphosphine oxide (TOPO) on the surface of CdSe/ZnS quantum dots (QDs) with various hydrophilic ligands causes a significant decrease in quantum efficiencies. Furthermore, the new ligands tend to desorb gradually from the nanoparticles, leading to aggregation and precipitation of the nanoparticles. More importantly, if more than one type of nanoparticles is needed for achieving multiple functions, each must be surface-treated separately before assembly. Therefore, it is highly desirable to develop a simple and general approach that allows the fabrication of multifunctional nanostructures by direct assembly of hydrophobic nanoparticles of various compositions. As their major applications are in the biomedical fields, it is also highly desired that these multifunctional systems be dispersible in water.